1. Field of the Invention
The invention relates to a device for contact-free potential measurements using a primary electron beam which triggers secondary electrons at the measuring point of an electronic component, preferably an integrated circuit, and the energy of the released secondary electrons are determined with a spectrometer.
2. Description of the Prior Art
It is known for testing the function of electronic component parts as, for example, integrated circuits, to have contact-free potential measurements which are accomplished with an electron microscope, preferably a scanning electron microscope. The electron microscope essentially consists of an electron-optical column which contains an electron gun and is generally provided with a device for keying the electron beam and for deflecting the electron beam. These devices are arranged in a vacuum within the column. The component to be tested is mounted outside of the electron-optical column in a test chamber which is also maintained at vacuum and in which the electron collector of the spectrometer is situated.
At the measuring point the primary electron beam releases secondary electrons from a component and the secondary electrons are collected by an electron collector and are converted into electrical signals. The number of secondary electrons released by the primary electron beam at the measuring point depends on the potential at the measuring point. For a quantitative potential measurement, the electron microscope is provided with a spectrometer, for example, a retarding field spectrometer which contains a cylindrical deflection capacitor that supplies the secondary electrons to the electron collector through a retarding field and the electron collector converts the electrons into a corresponding electrical signal and drives a controlled-gain amplifier. The output signal of the amplifier drives the spectrometer by way of a feedback loop. The grid voltage at the opposing field electrode is adjusted until the voltage between the grid and the measuring point has again achieved its original constant value. Then the change of the grid voltage directly corresponds to the potential change at the measuring point of the test site.
It is known that a function test of integrated circuits is possible with the circuits being mounted on a wafer and before they are provided with electrical connections. In these known methods, a so-called wafer prober which is provided with needles can be used to test the function of the components in that the needles are applied to predetermined parts of the surface, so-called pads. The surface units have an edge length of, 10-50 .mu.m. The measurement thus extends to parts of the component which are later provided with a terminal conductor mounted on the base. The electrical signals measured with these applied needles are supplied to a computer which monitors the function of the component ("Microanalytical Probing System" manufactured by POLYTEC).